Preservation of Wood Material by Chemical Techniques

Wooden Houses of Anatolia

Preservation of Wood Material by Chemical Techniques

INTRODUCTION 

More than 5000 products such as cellulose, varnishes, alcohol, synthetic fibres, sugars, plastics, adhesives, oils, dyes, soaps, inks, fodder, medicines, disinfectants, explosives, boards, many of which are known since centuries ago, are prepared from wood. Specially new wood products with better quality and properties are recently seen in the field of board products and composite materials. The demand for wood material and the price are seen to be increasing by time. 

Although wood material has many advantages with its unique properties compared with other materials, there are three deficiencies limiting its use:

a) Wood can be degraded by wood boring beetles, termites, wood destroying fungi and marine-borers because of its organic chemical structure. 
b) Wood can absorp water molecules with its free hydroxyl groups and its absorbed water content depends on the relative humidity of the air. Accordingly, three dimentions of wood change differently depending on the moisture content of the medium. 
c) Wood is a inflammable material. In order to give a longer service life and create new uses, this valuable product should be protected from being destroyed by the fungi, termites, the beetles, marine-borers, and fire, be given dimentional stabilization by treatment with water repellents and chemically cross linking, and be improved of some chemical processes. 

Timber preservation is becoming increasingly important: 

a) to help conserving forest resources; 
b) to protect wood with higher proportions of sapwood; 
c) to allow the use of non-durable wood species; 
d) to influence the use of alternative materials; 
e) to reduce the necessity for over demand; 
f) to provide economic and social benefits. 

WOOD PRESERVATIVES 

GENERAL CHARACTERISTICS OF WOOD PRESERVATIVES 

a) They must be toxic to fungi, pests and marine organisms. 
b) They must be free from objectionable properties in use and handling. 
c) They have to possess satisfactory properties with permanence under the conditions for which they are recommended for use. 
d) They should not have corrosive properties. 
e) They should not be expensive. 

Tar Oils: 

Creosote: The word creosote was first used to describe the oil produced from wood. Coal tar creosote is the oldest industrial wood preservative and has been used in enormous quantities for more than 150 years. Creosote is a brownish-black, oily liquid formed during the carbonisation of bituminous coal. The portion of the coal tar boiling between 200-400°C is creosote with a very complex chemical composition. Creosote contains hundreds of compounds, mainly hydrocarbons, with small amounts of tar acids and tar bases. Specifications are based on certain physical properties instead of chemical composition. Creosote is a very effective preservative, insoluble in water and therefore resistant to leaching, noncorrosive to metals, and has a high electrical resistance; it protects timber against splitting and weathering. Creosoted poles can last over 60 years, marine piles over 40 years, and railway sleepers a minimum of 30 years. Creosote is usually applied by an empty-cell process and occasionally by hot-and-cold open tank process. Since creosote gives off odours tainting foods and the volatile fractions are poisonous to plants, creosoted timber is not normally used in food containers and seed boxes. Creosote is not unsuitable for timbers to be painted. Creosote is not used for mining props because of fire hazard. Sometimes other chemicals have to be added to fortify the creosote and improve its performance. The addition of 2% pentachlorophenol eliminates the decaying of creosoted posts in the ground by Lentinus lepideus. Copper containing preservatives are added against the marine borer, Limnoria tripunctata. Small amounts of arsenic trioxide are added to improve the preservative properties against termite attack. Creosote loading is 400 kg/m3 in full-cell process and 140 kg/ m3 in empty-cell process (1-4). 

Carbolineum (Anthracene Oil): The tar oil containing higher proportions of high-boiling fractions is called carbolineum. It is generally applied by pressure impregnation, also by brush, spray or immersion; but a limited penetration is achieved. 

Carbolineum Avenarius: It is produced by the chlorination of carbolineum to achieve more effective preservation. 

Lignite oil: It is a tar oil derived from lignite. 

Peat tar: Peat tar is prepared and used in Russia and has similar properties to wood tar. 

Wood tar: One of the fractions of the tar derived from destructive distillation of wood is wood tar oil or creosote. Softwood tar known as Stockholm tar was at one time extensively produced and widely used for timber buildings by applying with brush. Although its preservative activity and persistence are lower, wood tar creosote was used at one time as a preservative for wood impregnation, giving excellent penetration, as its viscosity is rather lower than that of coal tar creosote. 

Shale tar: It is produced by distillation of bituminous shale tar. As recently as the tar was used for the impregnation of railway sleepers in Estonia and Lithuania. 

Petroleum products: These products are used as diluent for mixing with creosote. The P4 oil is the one specified for blending with creosote. 20-50 and 70-30 mixtures of creosote with petroleum are generally used. 

Oilborne Preservatives: 

Oilborne or organic solvent preservatives consist of active chemicals, an insecticide and/or a fungicide, dissolved in an organic solvent, such as a petroleum distillate. Of the millions of organic chemicals, only less than ten can be used as active ingredients in the formulations. Application of these chemicals gives long-lasting protection due to their natural insolubility in water. After evaporation of the organic solvent the active chemicals remain in the wood. 

Pentachlorophenol: Pentachlorophenol known as Penta or PCP is the most important and widely used fungicide of organic solvent preservatives. Commercial product manufactured by direct chlorination of phenol contains about 85% PCP. It is extremely toxic to fungi, insoluble in water and resist leaching, non-volatile, and non-corrosive to metals. 5% solution of PCP in heavy oils are used in the treatments. 

Lindane and Dieldrin: Lindane discovered in 1912 and used as insecticide since 1940s, one of the most important insecticides, does not accumulate in the environment. Dieldrin developed and used as insecticide in 1948 is persistent in environment. They are insoluble in water, chemically stable, and highly toxic to insects. Lindane is used as spray or dipping treatment of hardwood logs against Lyctus beetles in joinery treatments by immersion or double vacuum processes, and in situ remedial treatments against insect attack in buildings. Dieldrin is applied in joinery treatments for protection against termites and also used mainly as water-based dispersion for soil pretreatments against termites. It is used as 0.8% solution in petroleum solvent. 

Copper 8-quinolinolate: Copper 8-quinolinolate known as Copper-8 is relatively new preservative. It is manufactured by condensation of copper 8-quinolinolate and nickel 2-ethyl hexoate. Copper-8 is yellow-brown solid and made soluble in organic solvents by nickel 2-ethyl hexoate to give a green solution. It is toxic to wood pests except termites, but relatively harmless to animals and plants. This preservative is applied in wood material used for food containers, refrigerators, seed boxes, and greenhouse. Treatment solution should contain 0.045% Cu. 

Copper naphthenate: The preservative used first in 1920s as ‘Cuprinol’ gives dark-green waxy solution in organic solvents and waxy solution in organic solvents and waxy wood surface prevents over-painting. It is toxic to wood pests except termites and noncoroosive to iron or steel. Copper naphthenate is mainly used as paint-on preservative for boat maintenance. Treatment solutions contain 1-2% Cu. 

Bis (tri-n-butil tin) oxide: It is known as tributil tin oxide, TnBTO, or TBTO, excellent fungicide, more effective than PCP, insoluble in water, soluble in many organic solvents. TBTO has lower toxicity to humans than PCP. This preservative is mainly used as fungicide in joinery treatments and as a general preservative for boat maintenance. TBTO is applied as 0.5-1.0% solutions. 

Waterborne Preservatives: 

These are used in impregnation of mine props, domestic buildings, food containers, cooling towers. It is preferred for structural elements which are not to be painted and do not have any odour. Concentration of the solutions is about to 5%. 

Ammoniacal copper arsenite (ACA): It is known under the trade name of Chemonite with the composition of copper hydroxide (57.7%), arsenic trioxide (40.7%), and ammonia (1.5-2.0%). 

Acid copper chromate (ACC): This product known as Celcure consisted of copper sulphate (50%), sodiumdichromate (47.5%), and chromium trioxide (1.68%).

For CCA-Type A preservatives the trade name Greensalt has been applied to the product used for pole treatment and the name Erdalith to the product applied to lumber treatment. CCA-Type B is sold under the name of Boliden Salt K-33 and CCA-Type C under the name of Tanalith C and Celcure A. 

Chromated zinc chloride (CZC): The preservative is composed of sodium dichromate (18%) and zinc chloride (79.5%). 

Fluor-chrome-arsenate-phenol (FCAP): These Wolman type preservatives are mixtures of sodium fluoride and chromate, sodium arsenate and 2,4-dinitrophenol. 2,4-dinitrophenol has recently been replaced by sodium pentachlorophenate to eliminate the yellowing of treated timber. FCAP or Wolman type preservatives have been marketed under a wide number or formulations and trade names. These were Triolith, Minolith, Fluoxyth, Flunax, Tanalith U, Triolith U, Basilit U, Basilit UA, Osmolit U, Osmolith UA, Wolmanith U, Wolmanith UA, Trioxan U, Trioxan UA. The composition of FCAP Type A and Type B are given below (%). 

Other waterborne preservatives: Zinc chloride, zinc metaarsenite (ZMA), copperized chromated zinc chloride (CuCZC), chromated zinc arsenate (CZA), and copperized chromated zinc arsenate (CuCZA). 

Antiweathering Chemicals: When wood is exposed to the weather unprotected, its appearance soon deteriorates. Continuous wetting and drying causes cracking and splitting, ultra-violet light degrades and break down wood in the surface to give products which can be washed away by the rain. Also fungi and moulds growing in the cracks and splits causes the timber to appear dirty.

Paints and varnishes: Paints and varnishes gives the most effective means of maintaining the appearance of wood, provide that they completely cover the wood and they are not damaged in any way. The transparent film of varnish protects the wood from getting wet and screens the surface from damaging by ultra-violet light. Unfortunately, while these coatings give good protection against rainfall, they are unable to prevent changes in moisture content resulting from seasonal fluctuations in atmospheric relative humidity. As a result the painted wood will shrink or swell with changes in relative humidity, causing the surface coating to crack and split. Water penetrates into the wood and then staining fungi and moulds begin to colonise the surface. In a study conducted in England only 6% of over 200 varnishes tested presented unbroken protection for more than one year. Maintenance is generally required with expensive cleaning and rewarnishing. 

Water repellents and stabilisers: The term water repellent means the treatment coating the pores of structural material to prevent the absorption of water. Various waxes, specially paraffin waxes are the well-known water repellents used in wood preservative formulations. The aliphatic and aromatic hydrocarbon resins are inexpensive and efficient but solidify only by loss of solvent, redissolve by coating solvents. Natural drying oils, such as linseed oil can also be used. Alkyd resins can avoid the difficulties but they are very expensive. Generally, a mixture of waxes, hydrocarbon resins, and alkyd resins are used in order to prevent these problems. The organosilicon compounds are the best known water repellents but they posses many of the disadvantages of heavy organic oils and waxes. The silicones with a high degree of functionality to fix to the wood components are suitable to apply the wood giving good resistance to wetting failure. Organoaluminium compounds can incorporate unsaturated chains, and water repellent they can provide excellent adhesive bonding between the wood elements and alkyd systems. Commercial Manalox products are polyoxoaluminium systems. Formaldehyde treatment of wood in the presence of an acid catalyst will crosslink hydroxyl groups on adjacent chains, reducing the dimentions of wood and also the movement. Acetylation, the treatment of wood with acetic anhydride in the presence of a strong acid catalyst, considerably reduces the hygroscopicity of wood and increases the resistance to fungi. These chemical treatments are successful on condition that wood is completely impregnated. Impregnation of wood with a high retention of chemicals is called bulking. Some resin systems were used in systems were used in this way as in Impreg. Polyethylene glycol waxes, such as PEG, Carbowax, and MoDo, are also used in bulking. These systems are applied particularly for the stabilisation of archeological specimens and also floor blocks. Of the water repellent formulations, the Madison formula is the best known. The formula consists of paraffin wax, pigments and boiled linseed oil binder with pentachlorophenol and zinc stearate to give water repellency, colour retention and resistance to staining by fungi and moulds. Weather resistance can be improved by using a binder as in Madison formula. In the Royal process developed for the treatment of external joinery a waterborne treatment is followed by a deep treatment with a drying oil. 

Fire Retardants: 

Impregnation treatments: In 1905 a fire retardant called Minolith was introduced. This formulation consisted of triolith with a high concentration of rock salt to provide both a preservative and a fire retardant for use in the mines. Celcure F developed in 1930 contained boric acid instead of acetic acid and phosphates and zinc chloride. Minalith has the composition of 60% ammonium sulphate, 10% diammonium phosphate, 10% borax and 20% boric acid. Pyresote has also a composition of ammonium sulphate, boric acid and sodium sulphate, boric acid and sodium dichromate. Components of the typical fire retardants are leachable and hygroscopic. The most prefered components are ammonium phosphates, ammonium sulphate, zinc chloride, boric acid and borates. An American product called as Non-Com Exterior consists of a preservative which polymerises within the wood to give a non-corrosive product with good resistance to leaching. The full-cell process is used for treatment of fire retardants. Halogenated compounds such as chloronaphthalenes, chlorinated paraffins and bromophenols can be used with incorporated catalysts, like antimony oxide. 

Surface coatings: Surface coatings prevent flames spreading across the surface. These coatings are used in gymnasiums, hospitals, hotels, museums, restaurants, kitchens and laboratories. 

Intumescent coatings: When exposed to fire, these coatings soften and generates non-flammable gases. The coating trapping the gases bubble and produce a foam. Then the fire retardant solidifies to insulate the surface from the fire. 

Non-intumescent coatings: Some of them are formulated with materials which chemically interfere with the reactions of burning. Others based on silicates or borates melt in the fire and form a protective glassy film. 

Anti-stain Chemicals: Formulations of normal wood preservatives do not give a good control over sapstain fungi and superficial moulds that are responsible for staining in green wood and under coating systems in service. Sodium pentachlorophenate is found for use as an efficient chemical in spite of its high toxicity. Some co formulations based on sodium pentachlorophenate and borax have been used widely and the most popular consists of one part sodium pentachlorophenate and three parts borax. Pentabor has half the water of crystallisation removed in order to decrease the cost of transport. Trihalomethylthio compounds were also found efficient. Folpet (Fungitrol 11) has proved is very active. The dichlorofluorcompounds Fluorfolpet (Preventol A3) and Dichlofluanid (Preventol A4) are effective chemicals. The Madison formula with a pigmented and water-repellent composition was used as an anti-stain product. 

Preservatives preventing brown stain of beech, alder and hornbeam: 

Commercial chemicals such as Immutol B, Wolmanol-Buchenschutz, Xylamon ASR And Besileum were used to prevent brown stain occurring just after felling of the trees. Also a mixture of 85% pitch and 15% asphalt is used for preventing of cracking and splitting on cross-sections. 

LEACHING OF WOOD PRESERVATIVES 

The water-borne wood preservatives, resist leaching during service. Copper naphthenate resists also leaching because of relatively insolubility in water. The most rapid leaching occurs within the first months of service and is greatest in products with high retention levels and high proportions of exposed surface area. Leaching is increased by exposing the wood to high water flow, low pH and water soluble organic acids. 

PRESERVATION TREATMENT 

PREPARING WOOD FOR PRESERVATION TREATMENT 

Debarking: Some mills use water jets working at high pressure, the others use mechanical peeling. 

Machining: All handling of wood by hand or machine should be carried out before preservation process. First of all, the wood to be treated is machined to the required dimentions and the surface is handled in such a way that the wood is ready for treatment. 

Drying: Air-drying or kiln-drying is carried out for drying of wood.

Steaming: Steaming of timber in steaming vessels of plants enhance considerably the permeability of wood. 

Incising: Incising is making small slits or incisions in the wood of difficultly impregnable tree species to provide the penetration of preservation solution along the grain in two directions. 

Compression: Wood is passed through heavy rollers and the structure of compressed wood is changed to some extent that preservative liquid penetrates easily and uniformly. 

Ponding and sprinkling: Ponding and sprinkling enhance the absorption of preservative with the dissolution and enlargement of the pits by bacterial activity. 

PRESERVATION PROCESSES 

Preservation Treatments for Unseasoned Wood: 

Diffusion Processes 

Osmose method: The method used widely is osmose method. Highly water soluble and concentrated product is applied to the debarked surface of freshly felled and wet wood, generally poles. Poles are covered with an impermeable covering for 1-3 months to allow the diffusion process go successfully. The formulation applied for pine, spruce and fir contains water, NaF, dinitrophenol, starch and adhesive. 

Sap Displacement Processes 

Boucherie method: The well-known sap displacement method is applied to the freshly-felled unbarked poles. The capsules put on the thicker end of the boles are connected with pipes to a tank containing 1.5% copper sulphate. The preservative flowing from the tank at a higher place to the capsules takes place the sap of the boles in a few days. Other alternatives to this method are Gewecke Pressure and Suction Method and Slurry Seal Process. 

Nonpressure Processes: 

Brushing and Spraying: Brushing and spraying are the simplest methods for applying preservative chemicals. Only surface penetration of 1-5 mm can be achieved. 

Deluging: This is a treatment for sawn timber. Organic solvent preservatives is flooded over the wood surface while the timber is passing slowly through a short tunnel. 

Immersion: Immersion is a treatment of dipping timber in a tank containing preservative for from 5-10 seconds to 1-2 weeks. The application gives better results with higher diffusion rates than brushing, spraying and deluging. Short periods of immersion are ideal for treatments of joinery components. 

Hot and Cold Open Tank Treatment: The process is also known as Thermal Process. Hot preservative is pumped into the tank until the poles are completely immersed in the preservative solution for a period of six or longer hours. After the preservative is pumped from the treatment tank to storage tank the tank is flooded immediately with cold preservative solution. The cold solution brings about a partial vacuum in the wood cells and therefore more impregnation of the wood. 

High-Pressure Treatments: These are the most successful methods in wood preservation. Wood is treated with chemicals under high pressure in steel pressure wessel. 

The Full-cell Process (Bethell Process): The aim of the process is retaining maximum amount of preservative in the wood. Water-borne and also oil-borne chemicals are always applied by full-cell process. Creosote is only employed by this treatment when some special structural elements such as marine piling are treated with high retention rates of the preservative. There are five stages in Bethell process: 

a) Initial vacuum (635 mm Hg) for 15-60 minutes. 
b) Filling the vessel with preservative solution. 
c) Pressure (10-14 kp/cm2) for 1-6 hours. 
d) Draining preservative after releasing the pressure. 

The Empty-cell (Rüping) Processes: The methods were generally invented to reduce the amount of creosote used in the treatments with the same penetration. In the treatments with the same penetration. In this procedure there is no initial vacuum and a large amount of the creosote is expelled from the wood by the compressed air trapped within, leaving the cell walls thoroughly treated. 

There are five stages in the process: 

a) Initial air pressure (4 kp/cm2). 
b) Filling the vessel with preservative. 
c) Pressure (10-14 kp/cm2) period for 1-3 hours. 
d) Draining preservative after releasing the pressure. 
e) Final vacuum (600 mm Hg, 10 minutes). 

Lowry process: This method differs from the Rüping process in hat preservative is pumped into the vessel against atmospheric pressure. No initial vacuum or pressure is applied and less solution is forced out of the wood than with the Rüping treatment. 

Oscillating Pressure Method (OPM): The difficulties encountered using the Bethell process for treatment of very resistant wood species lead to the use of repeated cycles of vacuum and pressure with the improvement of penetration. The high pressure is 8 kp/cm2 and the vacuum 720 mm Hg. Green or seasoned timber is treated with water-borne chemicals, generally CCA formulations. The method is specially applied to the poles of resistant species such as spruce and fir. 

Alternating Pressure Method (APM): In this modified method the alternating pressure changes within each cycle from 8 kp/cm2 to atmospheric pressure. Also green and difficultly impregnated wood can be treated by the process, eliminating drying of the timber. 

Ultra-high Pressure Method (HP): A full-cell process using a pressure of about 70 kp/cm2 is introduced with the aim of improving the penetration and retention of preservative in eucalpt species which is difficult to impregnate by other methods. 

Low-Pressure Treatments: 

Double Vacuum Process: The treatment has remarkable industrial success in the United Kingdom with hundreds of plants in operation. Since the timber can be glued, painted or glazed in a few days after the treatment, the process is well suited to the needs of joinery industry. There are five stages of treatment. 

a) Initial vacuum of 250 mm Hg (3 minutes) for pine and 625 mm Hg (10 minutes) for spruce. 
b) Filling the vessel (rectangular or circular cross-section) with usually an organic solvent type preservative solution. 
c) Pressure of about to 2 kp/cm2, 3 minutes for pine and one hour for spruce. 
d) Drainig the preservative after the pressure is released. 
e) Final vacuum of 500 mm Hg for 20 minutes. 

In situ Remedial Methods Bandage Method: Ready-made bandages containing Pol-Nu Type and Wolmanit-TS are placed on the transmission poles at the ground line to control decaying and extend service life. 

Cobra Process: The process was developed as a remedial treatment also for transmissin poles at the ground line. Generally a Wolman type salt is forced through a needle into the pole. 

Drilled Hole Method: The method is applied to wood structures with a high risk of decay such as bridges and piles in water. The holes drilled at a diameter of 15-25 mm is filled with solid preservative and closed to allow the chemical impregnate the wood by diffusion. 

PROPERTIES OF TREATED WOOD AFFECTING USES 

Strength: Waterborne preservative treatments generally reduce the mechanical properties of wood. The treatment does not reduce the load carrying capacity to below acceptable levels. Incising may cause a slight decrease in strength but it gives an increased protection. Unless the steaming treatment period kept as short as possible, serious weakening of the wood can be observed. High pressure can brings about collapse of wood cells, especially with woods of low density. When the wood is treated to accepted chemical loading by normal industrial preservation methods any significant losses of strength is observed.

Flammability: Wood processed with water-borne salts has no greater flammability. However the wood treated freshly with creosote or heavy oil mixtures presents greater fire hazard. Therefore, mine props are processed with water-borne salts. After a few month creosote treated wood presents no fire hazard. 

Electrical Conductivity: Creosote and organic solvent preservatives have no effect on the conductivity. Although water-borne chemicals change the electrical conductivity slightly, the differences are small and can be ignored for practical purposes. 

Safety 

Domestic and Industrial Buildings and uses: Creosote treated timber is not normally used in domestic buildings because of unpleasant and irritating odour. Timber used for domestic purposes is treated with water-borne chemicals by using pressure treatment or with organic solvent preservatives by double vacuum method. Wood treated with both creosote and water-borne preservatives is used as transmission poles, in warehouses and industrial and agricultural buildings. 

Greenhouses, Seed and Mushroom Boxes: The timber treated with creosote or PCP are not recommended and the CCA processed timber can be used. 

Children Playground Equipment and Garden Toys: Water-borne preservatives fixed in the wood can be used with absolute safety. Deposits seen on the surface are removed by redrying the wood to 22% moisture content, hosing down and drying again. Two coats of a water-repellent finish are also recommended as precaution. Creosoted wood is unsuitable. 

Animal Pens: Most of the preservatives can be used with safety for animal pens. Timber treated with creosote should be air-dried and deposits on the timber treated with water-borne salts should be removed as described above. The use of PCP in preservatives should be avoided. 

Food Containers: Creosoted timber must not be used as food containers. Copper 8-quinolinolate is recommended for the containers. The preservatives fixed in the wood, such as CCA, can be used with absolute safety on condition that surface deposits should be removed as described previously. 

DIMENTIONAL STABILITY AND CHEMICAL MODIFICATION OF WOOD 

One of the disadvantages of wood is dimentional instability by the changes in relative humidity of air. To prevent the different changes in different dimentions and improve some properties wood is treated with. 

a) water-repellent formulations 
b) phenolic resins 
c) polyetlhylene glycol 
d) monomers polymerising in the wood (wood-polymer composites) 
e) acetic anhydride with catalyzer for acetylation of the material. 

Prof. Dr. Harzemşah HAFIZOĞLU 
Z.K.Ü., Bartın Faculty of Forestry, Prof. Dr.

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